1. Field of the Invention
The present invention relates to processes for the deposition or etching of films on substrates, especially on the fabrication of multilayer systems.
The present invention also relates to devices for the deposition or etching of films on substrates, especially for the fabrication of multilayer systems in particular such devices comprising a substrate holder and at least one deposition source with target.
2. Description of the Related Art
Reflecting multilayer structures for the reflection of short wavelength electromagnetic rays are used e.g. in semiconductor lithography. They are used especially in the extreme ultraviolet and soft x-ray wavelength range. The extreme ultraviolet wavelength range (EUV) is the transition range between the ultraviolet and the soft x-ray range and generally comprises the wavelengths from approximately 16 nm to approximately 10 nm. The soft x-ray range generally comprises the wavelength from approximately 10 nm to 1 nm. For example, in EUV lithography wavelengths of approximately 13 nm are, in particular, used.
The simplest multilayer systems consist of alternating layers of two different materials, i.e. an absorbing and a reflecting material. More sophisticated multilayer systems can consist of more than two materials, the layer thickness and thickness ratios being constant or varying over the multilayer depth and/or plane. The principle of operation of multilayer systems is that the intensity of radiation which is reflected at different boundaries is increased by constructive interference if the layer period is made to match the wavelength of the incident radiation.
For the EUV range, mostly multilayer systems based on alternating layers of molybdenum and silicon are used. Theoretically, such systems can reflect up to 75% at near normal incidence of an incident wavelength of 13.5 nm. To the current state, practically achievable reflectivity using different deposition methods reaches 69.5 to 0.70.0% due to imperfections in layer manufacturing. The main limiting factors are the formation of interfacial roughness, intermixing of adjacent layers, contamination of layers, thickness errors and deviation of the density of deposited materials from bulk densities.
Mo/Si multilayers with a state-of-the-art performance have so far been fabricated by electron beam evaporation and magnetron sputtering.
Electron beam evaporation is a technique based on heating material in a crucible by a focused beam of high energy electrons. Electron beam evaporation features a low kinetic energy atomic flux (approximately 0.1 eV) and, especially when combined with ion assistance and/or post deposition polishing, it allows to selectively control the energy contribution at every stage of the film growth. Thus, the amount and the moment of applying additional energy to the layer system is controlled independently and allows selective optimization during the different stages of the layer growth. The particles have energies around 10−2-1 eV and only a small number of ions is produced in evaporant materials by electron bombardment.
Sputtering is a technique based on the ejection of target material by bombardment with energetic rare-gas ions, produced in a discharge or by a separate ion source. The gas used is also called working gas. The magnetron sputtering was developed in order to increase the sputter rate, where the magnetic field is used to trap electrons near the target surface, resulting in an increased ionization efficiency of the working gas (e.g. argon or krypton) and an increased amount of ionized particles bombarding the target. An essential feature of the sputter process is that it allows a particle flux of which the composition is identical to the target materials, i.e. compound materials like e.g. B4C are not dissociated in the creation of the particle flux, as is the case in most other deposition processes. Magnetron sputtering can thus be used for a wide range of materials and offers good control of the lateral profile of the films. In addition, magnetron sources exist in technical realizations that are compatible with the demands of ultrahigh vacuum systems.
To improve the quality of the deposited layer with respect to roughness and stability, the deposition can be ion assisted, i.e. the deposited layer is bombarded simultaneously with ions, or the layer surface can be etched by ion bombardment after deposition. For these methods, additional ion sources are used besides the physical sources supplied by e.g. electron evaporation or sputtering.